A river basin in southern Asia is so enormous that 750 million people rely on it for their groundwater. Now, a new study in Nature Geoscience presents an equally staggering stat: 60% of that water is unfit for drinking or farming because it's contaminated by salt or arsenic, reports the Guardian.

About 23% of the groundwater in the Indo-Gangetic Basin to a depth of 650 feet is too salty—an issue perhaps caused by poor farmland irrigation or drainage—and another 37% is tainted by toxic levels of arsenic, researchers say.

Like salt, arsenic is present naturally, but levels can spike with mining and the use of fertilizers. The basin, so named because it's near the Indus and Ganges rivers, serves people in Pakistan, India, Nepal, and Bangladesh, and it accounts for one-quarter of the world's groundwater.

"Elevated arsenic is primarily a concern for drinking water, while salinity affects irrigation and also the acceptability of groundwater for drinking," the researchers say, per International Business Times.

However, researchers say that "deep tube wells" could penetrate deeper into the basin to pull out non-contaminated water, per Nature World News. Perhaps the only good news from the report is that the amount of water in the basin remained relatively stable, in contrast to other groundwater sources around the world.

(Testing has found issues with 2,000 US water systems.)

This article originally appeared on Newser: One of Our Largest Water Sources Is Contaminated